1. Field of the Invention
The present invention relates to missiles. More specifically, the present invention relates to roll control in canard-controlled missiles.
2. Description of the Related Art
Future concepts for highly maneuverable missiles require active control of body roll. This has traditionally been accomplished with a cruciform arrangement of control surfaces, with four separate actuator motors moving the fins to achieve control through application of aerodynamic forces. Active control of roll has been largely limited to tail-control airframes, which have a restricted volume in the area around the rocket motor nozzle to package actuators. Tail control airframes are less desirable for high maneuverability applications, since they have significant limitations in their speed of response by virtue of the tails being behind the center of gravity. The rapid maneuver response of canard-controlled airframes, a result of locating the control surfaces forward of the center of gravity is more desirable for high maneuver applications; however, roll control via canards has seen limited exploitation because of well-known canard-tail interaction problems.
Roll control in a canard-controlled airframe has been attempted in two ways. The first approach allows the tail assembly to freely roll on a bearing. The tails can exert pitch and yaw forces, but adverse roll from the canard downwash is eliminated by virtue of the roll bearing. Allowing the tail to freely roll eliminates roll coupling, but causes problems in hysteresis and stability. Hysteresis occurs when the tail stops rolling depending on how a particular flight condition was reached. The resulting stability is therefore flight condition path dependent. In addition, as the tail rolls, the aerodynamic effectiveness of the surfaces changes, so the stability shifts according to the tail roll rate. These effects complicate autopilot design and cause restrictive bounds to be put on lateral g capability, limiting the maximum maneuver capability of the system. The second approach to decouple canard pitch control from tail roll effects is to put separate actuators in the tail section, allowing them to command tail deflections and overpower the canard downwash effects. This approach requires packaging of conventional actuator motors in the aft end of the missile. In addition, the tail surfaces are rotated in a conventional manner, with associated free play and gear train complexity for each fin. Furthermore, the size of fins designed for roll control will differ from that designed for pitch and yaw stability, resulting in a less than optimal compromise which ultimately means less maneuverability.
Hence, a need exists in the art for an improved system or method for controlling body roll in canard-controlled airframes which offers greater performance potential than has been achieved by the prior art.